Fuel cleanliness requirements for fuel injection systems are extremely demanding in order to ensure reliable and robust engine performance. To a large degree, fuel filtration is responsible for meeting the fuel cleanliness requirements associated with the control of abrasive particles and non-dissolved water. However, the presence of soaps in fuel can cause injector sticking, and in extreme cases may even plug fine filters. Soaps, for purposes of this disclosure, are defined as any chemical species that can be measured using American Oil Chemist's Society (AOCS) standard Cc17-95 Soap in Oil Titrimetric Method (American Oil Chemist's Society (AOCS) standard Cc17-95 Soap in Oil Titrimetric Method) adapted for use in fuel. Typically, soaps are metal carboxylates. They may be present as semi-solids or soft contaminants at ambient temperatures, or as dissolved species that may become solids under temperature and pressure conditions found in fuel systems. Soap issues are typically the result of the use of carboxylic acid based fuel lubricity enhancers or corrosion inhibitors used to protect pipelines. When fuel is contaminated with metal ions, such as those that may be carried in with water or lube oil contamination, hydrogen ions are exchanged for metal ions and soaps are formed. Basic moieties found in diesel fuel, such as those resulting from the fuel refinery processes or marine transport, e.g., NaOH, Ca(OH)2, or lube oil contamination, react with the carboxylic acid to remove the hydrogen ion, resulting in a water molecule and metal carboxylate. Soaps are less soluble than the parent carboxylic acids, and they may also reduce the effectiveness of the corrosion inhibitor additive. To a large extent, soap issues have been dealt with by attempts to improve the quality of bulk fuel prior to delivery and through the use of additives that clean soap deposits from injectors or prevent their deposition. However, these methods are not always effective or practical and hence, there is a desire for a filter solution to the soap problem. Preferably, this would be in the form of filtration on-board the engine or vehicle, or at the point-of-use/delivery to the engine fuel tank.
Under field conditions, conventional on-board fuel filtration does not remove soap nor eliminate injector sticking issues. It has been reported that semi-solid metal carboxylates have plugged high efficiency bulk fuel tank filters (Steven R. Westbrook, James Doyle, Philip Johnson, “Analysis and Identification of Contaminants in Diesel Fuel Filtration and Storage Systems,” Proc. 10th International Filtration Conference, September 2010) and have been found in ultra-high efficiency advanced fuel filters (Mark Wieczorek, William Haberkamp, Barry Verdegan, “NEXT GENERATION DIESEL FUEL FILTER,” Proc. World Filtration Congress 11, April 2012). Despite this, one would not expect this approach to eliminate soap issues, since the problems occur at such low concentration levels. Sodium concentrations that are less than 0.1 ppm Na typically do not cause issues, while concentrations as low as 1 ppm and higher are known to cause sticking. For concentrations between 0.1 and 1 ppm Na, problems may or may not occur depending on the nature of the carboxylic acid and its concentration. To put this in perspective, 1 ppm Na present as the salt of hexadecenyl succinic acid, a corrosion inhibitor known to cause injector deposits, is enough to form one ˜3 μm soap particle per mL of fuel. This is smaller than the 6 to 15 μm particles that conventional fuel filters are designed to remove. Indeed, it is smaller than the particle size that the particle counters used in the industry for contamination control are even able to detect. It is noteworthy that for other metals, such as Pb, Zn and Cu, issues occur at even lower concentrations (WO 2010/003504 A1 Removal of Metal from Diesel Fuel). As a practical matter, much of the metal carboxylate present at these concentrations remains soluble at ambient conditions. In engine applications, soap formation is most commonly associated with the presence of Na, K, Ca, or Mg ions, although others, including Pb, Zn, Cu, and Fe, may also cause issues. Thus, one would not expect conventional filtration to offer a viable solution to the soap problem.
WO 2010/003504 A1 describes the use of ion exchange resins to remove metal ions from fuel. One would expect that soap would not form if the metal ions were removed. In this application, the ability to remove the selected metal was demonstrated. However, the lubricity of the fuel was not improved and in some cases was actually decreased by ion exchange. A significant shortcoming of the method is that, it does not address the removal of soap that was previously formed, notably semi-solid soaps. In this form, the metal is not present in ionic form, and therefore, not amenable to removal by ion exchange. In practical applications, including on-board filtration, the soaps form upstream of the removal process at the point of metal contamination, hence are already present.